The grinding of material such as coal or clay in the usual grinding apparatus is frequently found to produce an excessive percentage of extremely fine particles which are difficult to handle. When such material contains moisture of the order of 15% or more the tendency for the coarse and fine particles to agglomerate is encouraged, with the result that the return of coarse particles to be reground in a grinding apparatus will be accompanied by fines sticking to the coarse particles. Thus, the fines are further reduced in size and the apparatus has its efficiency reduced because of the amount of material being rehandled and reground. A further problem with regrinding fines beyond the desired cut point (size) is the reduction in efficiency of the operating apparatus down stream in the furher processing.
The problem in reducing coal to a fineness in the range where approximately 70 percent will pass a 200 mesh screen, so as to make it suitable for coal gasification, is to prevent the coal from being reduced to an extreme fineness of the order of 44 microns or finer, because the fines easily escape the gasification processing apparatus and are lost.
The problem in reducing clay to a condition suitable for use as oil well drilling mud or in iron ore pellitizing, is to prevent reduction to extreme fineness. When clay contains as much as 30% to 40% moisture it is difficult to reduce it to a substantially uniform size. In addition to the moisture problem, extreme clay fines have a tendency to float and when placed in a container there is a period of time needed for the particles to settle down and compact so that a reasonable shipping weight can be reached. The waiting period is expensive and ties up shipping containers.